Escherichia coli DNA gyrase contains at least two subunits. Subunit A is the nalA gene product (the nalidixic acid target protein) and subunit B is probably the cou gene product (the novobiocin target protein). We hope to establish directly the structural gene for subunit B and the precise stoichiometry of the gyrase subunits. The addition of nalidixic acid to a DNA gyrase reaction mixture leads to accumulation of a stable enzyme-DNA complex which we believe is an intermediate in the gyrase reaction. The gyrase subunit attached to DNA and the bond holding the complex together will be determined. Finally, the subunits which can bind labeled novobiocin or labeled nalidixic acid will be determined.